Photomultiplier Tube

A vacuum tube that uses the photoelectric effect and secondary emission to detect and amplify low levels of light. It works by converting incident photons into electrons and then multiplying those electrons through a cascade of dynodes to produce a measurable current.

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Technical Description

The PMT consists of a photocathode that emits photoelectrons when struck by photons. These electrons are accelerated toward a series of dynodes, each maintained at progressively higher potentials. At each dynode, secondary electrons are emitted, amplifying the signal in a chain reaction until a final measurable current is collected at the anode.

Core Components

Photocathode: Converts incident photons into photoelectrons via the photoelectric effect.
Dynodes: Electrode stages that multiply electrons through secondary emission.
Anode: Collects the multiplied electrons to produce output current.
Vacuum Envelope: Maintains a vacuum environment for free electron movement.
Voltage Divider: Provides the required bias voltages to dynodes.

Specifications

Parameter	Typical Value
Wavelength Sensitivity	160 – 900 nm
Gain	10⁶ – 10⁷
Response Time	< 3 ns
Operating Voltage	800 – 1500 V
Dark Current	< 1 nA

Advantages

Extremely high sensitivity to light
Fast response time
Low noise and excellent signal-to-noise ratio

Applications

Scintillation detectors in nuclear and particle physics
Medical imaging (e.g., PET scanners)
Fluorescence spectroscopy
Low-light level detection in astrophysics

Safety Notes

Handle high-voltage circuits with caution.
Protect PMTs from exposure to strong light, which may damage the photocathode.
Ensure vacuum integrity to avoid implosion risk.